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Clay dewatering; smectite dehydration

Left: Clusters of interlocking, flaky, authigenic smectite; a top-down view of the cemented pore space (beneath the twinned calcite), and at the level of the 10um bar scale a cross-section view of the same mineral showing a kind of radial geometry. The cruciform crystal is calcite that was a later precipitate than the smectite. Image Credit: Anthony Priestas, Boston University. Right: Flaky, mixed layer illite (75%) – smectite (25%) from a Silurian bentonite, Woodbury Quarry, Worcestershire. Both images from Images of Clay Archive of the Mineralogical Society of Great Britain & Ireland and The Clay Minerals Society and the Hutton Institute.
Left: Clusters of interlocking, flaky, authigenic smectite; a top-down view of the cemented pore space (beneath the twinned calcite), and at the level of the 10um bar scale a cross-section view of the same mineral showing a kind of radial geometry. The cruciform crystal is calcite that was a later precipitate than the smectite. Image Credit: Anthony Priestas, Boston University. Right: Flaky, mixed layer illite (75%) – smectite (25%) from a Silurian bentonite, Woodbury Quarry, Worcestershire. Both images from Images of Clay Archive of the Mineralogical Society of Great Britain & Ireland and The Clay Minerals Society and the Hutton Institute.

A series on clay minerals – their crystal chemistry, identification, formation, diagenesis

It has long been recognized that mudrock compaction is an important driver of fluid flow in sedimentary basins (the term mudrock refers to clay-silt dominated lithologies, from mudstone to shale). Interest in shale dewatering was driven in part by developing models of hydrocarbon migration. The earliest hypotheses of compaction and sediment dewatering attributed depth-dependent porosity decrease (and density increase) to the weight of the overlying sediment pile; H. Hedberg, (1926) referred to this as gravitational compaction. Seminal papers by M. Powers (1967) and H. Burst (1969) challenged the assumption of gravitational dominance, and introduced the idea that mineral diagenesis also plays an important role promoting sediment compaction and dewatering. The Powers, Burst, and later proponents of these models (e.g., Foscolos and Powell 1979, 1980, Foscolos, 1984, PDF;  Curtis, 1985, PDF)  focused on clay diagenesis, particularly the smectite group of expanding clays. Their models also pointed out the fundamental differences in how fluid escaped during the mechanical and ‘diagenetic’ stages of compaction.

Mechanical compaction

Physical compaction of sediment begins soon after deposition, driven by the weight of the overlying sediment. The mechanism involves compressive stresses pushing the granular or particle frameworks into closer packing arrangements. Mechanically weak granular components like lithic fragments will tend to flatten. There is a concomitant increase in density, a decrease in effective porosity, and importantly the expulsion of interstitial water. These variables continue to evolve as burial depths increase. Under these conditions, water expulsion and mass transport of dissolved solids is primarily advective. Advective flow will continue until there is little remaining effective porosity, at which point diffusive flow will become increasingly important.

The condition that water is expelled during compaction can be restated. – if interstitial water cannot escape, then physical compaction will not take place. If the pathways for fluid expulsion are blocked, then fluid pressures will increase with burial depth and porosity will be preserved; a condition called compaction disequilibrium. Fluid pressures may increase to the point where they exceed rock strength, the overpressured domain will breach and fluid will escape, mixing with local flow aquifer systems.

A complicating factor associated with decreasing porosity and permeability with burial depth is that it becomes progressively more difficult for water to escape, particularly by advective flow. Chemical diagenesis accounts for some of this porosity loss attendant on precipitation of pore-filling cements like calcite, dolomite, and quartz.  However, none of these cements results in additional mechanical compaction; in fact, for arenites and coarser-grained rocks these cements may strengthen the rock framework and inhibit further compaction. The decrease in the mechanical component of compaction is partly offset by ‘diagenetic’ compaction – this applies mostly to mudrocks, specifically to the dehydration of expandable clays and the smectite-illite transformation. There is also evidence that excess fluid pressures can develop during diagenetic smectite dehydration – based on measurement of salinities and 18O stable isotopes (e.g., Tremosa et al., 2021, PDF).

Smectite dehydration

Smectite dehydration refers to the removal of interlayer water molecules in the smectite (expandable clay) crystal lattice.  The general picture of clay dehydration and diagenesis has been established from analysis of borehole rock and fluid samples (for clay minerals this includes XRD and electron microscopy), experimentation, and geochemical modeling. Evidence of anomalously low Cl– concentrations in some deep wells is also important because it indicates dilution of interstitial fluids at depth – this is referred to as fluid freshening that is usually attributed to the expulsion of lattice-bound water in expandable clays.

The compaction models developed by Powers, Burst (op cit.) and others show the stepwise transition from purely mechanical compaction to combined mechanical-diagenetic compaction at depth. Their graphical plots show relatively abrupt increases in water expulsion above those predicted from normal lithostatic pressures – Stages II and III in the Perry and Hower (1972) model are equated with expandable clay dehydration.  These authors allude to the collapse of expandable clay lattices (specifically smectite, vermiculite) resulting from the loss of lattice-bound water. The changes in smectite lattice dimensions as dehydration proceeds can be calculated from basal d-spacings in X-ray diffractograms.

Mudrock dewatering – depth plots compiled by Perry and Hower (1972, op. cit.) (references in the text). The earliest stage is dominated by mechanical compaction, loss of porosity, and coincident dewatering. In these models, loss of interlayer water in expandable clays in subsequent stages is significant at depths and temperatures where chemical diagenesis becomes important.
Mudrock dewatering – depth plots compiled by Perry and Hower (1972, op. cit.) (references in the text). The earliest stage is dominated by mechanical compaction, loss of porosity, and coincident dewatering. In these models, loss of interlayer water in expandable clays in subsequent stages is significant at depths and temperatures where chemical diagenesis becomes important.

The staged dehydration of smectite depicted by these models is inextricably linked to its conversion to mixed smectite-illite and illite clays during burial diagenesis; the reactions are temperature dependent – they also depend on the availability of potassium. It is generally assumed that K is derived from dissolution of detrital potassium feldspar (e.g., Curtis, 1985, op. cit.).

However, more recent experimentation of clay consolidation shows the dehydration process is more complicated (Hüpers and Kopf, 2012. OA).  Clay samples subjected to normal compressional stress at constant temperature, released interlayer water at pressures as low as 1.3 MPa, but without any change in the clay mineral chemistry. In these experiments the initial fluid was standard seawater. The results were confirmed by XRD where smectite basal (crystal) d-spacings decreased with increasing consolidation, and water analyses showed significant dilution of seawater, confirming fluid freshening. Samples were tested to 70 MPa, which corresponds to burial depths of about 3 km under normal lithostatic pressures. The experiments demonstrated that clay lattice collapse due to compressional dehydration could release up to 17% of the total fluid expelled during mechanical compaction.

Left: A schematic of smectite crystal structure showing the response to reduction in interlayer water molecules (blue) in XRD- d-spacings (for lattice basal planes). Right: A plot of measured smectite interlayer d-spacings in samples subjected to experimental compressional stresses. The maximum pressure was 70 MPa. D-spacings begin to change at pressures greater than 1.3 MPa. Both figures modified from Hüpers et al., (2012 op. cit.) Figure 1 (left) and Figure 3d.
Left: A schematic of smectite crystal structure showing the response to reduction in interlayer water molecules (blue) in XRD- d-spacings (for lattice basal planes). Right: A plot of measured smectite interlayer d-spacings in samples subjected to experimental compressional stresses. The maximum pressure was 70 MPa. D-spacings begin to change at pressures greater than 1.3 MPa. Both figures modified from Hüpers et al., (2012 op. cit.) Figure 1 (left) and Figure 3d.

Two plots from the Hüpers et al., (2012) compression experiments on smectite, showing the fraction of water released from the clay interlayers during compression (left), and the change in salinity (represented by chloride) with increasing compression (right). The starting water composition was standard seawater. Dilution (freshening) begins at 1.3 MPa, corresponding to very shallow burial. Fluid freshening at 70 MPa has reduced the chloride content by 45%. Both figures modified from Hüpers et al., (2012 op. cit.) Figure 3c (left) and Figure 3a.
Two plots from the Hüpers et al., (2012) compression experiments on smectite, showing the fraction of water released from the clay interlayers during compression (left), and the change in salinity (represented by chloride) with increasing compression (right). The starting water composition was standard seawater. Dilution (freshening) begins at 1.3 MPa, corresponding to very shallow burial. Fluid freshening at 70 MPa has reduced the chloride content by 45%. Both figures modified from Hüpers et al., (2012 op. cit.) Figure 3c (left) and Figure 3a.

Restating the conditions for mudrock compaction

Our current state of knowledge indicates that compaction and dewatering of clay-rich sediment occurs in three ways – we need to consider the relative contribution of each process when deciphering standard porosity-depth, or compaction curves for mudrocks (like the example shown here):

Shale porosity-depth relationships (compaction) from multiple wells, plotted for simplicity as an envelope. The generalized contribution of each compaction mechanism is indicated with arrows. (Modified from Allen and Allen, 2005, Figure 9.3b)
  1. Mechanical compaction and expulsion of interstitial water due to sediment loading that is responsible for 35% to 85% reduction in mudrock porosity at depths less than a kilometre.
  2. Loss of smectite interlayer water and accompanying lattice collapse due to normal compressional stress.
  3. Loss of interlayer water during chemically induced smectite-illite transformations. There is general agreement that the conversion of smectite to illite takes place through intermediate mixed-layer smectite-illite clays. Two competing hypotheses explain how this transformation occurs; both use the same kind of data – borehole rock and fluid samples (salinity, cation ratios etc.), XRD (to decipher crystal structure), and in more recent studies, various mass spectrometry tools.
    • The standard hypothesis with various modifications over the last few decades involves a relatively homogeneous transition from authigenic smectite to randomly ordered smectite-illite, where the degree of crystal lattice ordering and the proportion of illite generally increases with depth. Clay transformation begins in earnest at about 80o C, which corresponds to the lower temperature limit for hydrocarbon generation, and precipitation of cements like quartz and dolomite (see the references cited above). The SiO42- released by these clay reactions is repurposed as quartz cement or authigenic albite (e.g., Hutcheon et al.,, 1980).
    • An hypothesis that posits heterogeneous transformations where authigenic smectite, mixed layer smectite-illite, illite, and kaolinite coexist (Lanson et al., 2009, OA).  Randomly ordered authigenic smectite-illite with illite content as high as 50%, can occur at depths of a kilometre and less, and coexists with discrete authigenic smectite. Like the preceding hypothesis, lattice ordering and the proportion of illite in the mixed layer phases increases with depth, but different intermediate phases can coexist. Lansen et al., found that discrete authigenic smectite can still be found at depths to 4000 m and according to the geothermal gradients they cite, to temperatures of about 135o C.

References not linked in the text

A.E. Foscolos and T.G. Powell, 1979. Catagenesis in shales and occurrence of authigenic clays in sandstones, North Sabine H-49 well, Canadian Arctic Islands. Canadian Journal of Earth Science, v. 16, p. 1309-1314.

A.E. Foscolos and T.G. Powell, 1980. Mineralogical and geochemical transformation of clays during burial diagenesis and their relation to oil generation. Canadian Society of Petroleum Geologists, Memoir 6, p. 153-172.

M.C. Power, 1967. Fluid release mechanism in compacting marine mudrocks and their importance in oil exploration. AAPG Bulletin v. 51, 1240-1253.

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